Long-Term Evolution (LTE) system supports two duplex modes including Frequency Division Duplex (FDD) and Time Division Duplex (TDD). FIG. 1 shows a frame structure of a TDD system. Each radio frame is of 10 ms length and is divided into two 5 ms half-frames. Each half-frame includes eight 0.5 ms slots and three special fields, i.e., downlink pilot slot (DwPTS), guard period (GP) and uplink pilot slot (UpPTS). The total length of the three special fields is 1 ms. The TDD system supports 7 kinds of uplink-downlink configurations, as shown in Table 1. Herein, D denotes downlink subframe, U denotes uplink subframe, and S denotes a special subframe including the above three special fields.
TABLE 1Table 1 uplink-downlink configurations of LTE TDDConfig-SwitchingurationpointSubframe indexindexperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
In the LTE-Advanced (LTE-A) system, a wider working bandwidth is obtained through combining multiple Component Carriers (CC) via a Carrier Aggregation (CA) technique, and therefore data transmission rate may be further increased. Each CC corresponds to one Cell. According to current LTE standard (Rel-12), a UE may work on at most 5 CCs at the same time, wherein one of them is a Primary Cell (Pcell), and other CCs are Secondary Cells (Scells).
In downlink communication of the LTE-A system, Hybrid Automatic Repeat reQuest (HARQ) technique is utilized to ensure reliability of downlink data receiving. The UE receives a DL-GRANT, wherein the DL-GRANT is carried by a Physical Downlink Control Channel (PDCCH) or an Enhanced Physical Downlink Control Channel (EPDCCH). Physical Downlink Shared Channel (PDSCH) is received according to indication information in the DL-GRANT. For each Transmission Block (TB) received via PDSCH, or received Physical Downlink Control Channel indicating release of semi-persistent scheduling (hereinafter the above two are referred to as downlink HARQ transmission), the UE needs to feed back ACK (correct receiving) bit or NACK (incorrect receiving or lost) bit to the base station via corresponding uplink subframe, hereinafter referred to as HARQ-ACK bit. If the base station receives the NACK bit, the base station re-transmits the TB corresponding to the NACK or the PDCCH indicating the release of the SPS. According to different HARQ-ACK mechanisms, the LTE-A standard defines corresponding method for determining the number of HARQ-ACK bits to be fed back and values of the HARQ-ACK bits.
In a FDD system, if the UE feeds back HARQ-ACK bit in an uplink subframe n via a Physical Uplink Shared Channel (PUSCH), the number of HARQ-ACK bits to be fed back is determined according to the number of carriers configured for the UE and a transmission mode (transmission mode of one TB or transmission mode of two TBs) of each carrier. For each carrier configured for the UE, if the transmission mode is one TB, the carrier corresponds to one HARQ-ACK bit. If the transmission mode is two TBs, the carrier corresponds to two HARQ-ACK bits. The bits are arranged according to an ascending order of the indexes of the carriers, to form a HARQ-ACK bit sequence that the UE finally feeds back in uplink subframe n (the HARQ-ACK bit sequence refers to that before channel coding, the same applies hereinafter). Subsequently, it can be seen that in HARQ-ACK feedback mechanism defined by current standard, the HARQ-ACK sequence finally fed back by the UE may depend on configured carrier number and TM. Thus, the HARQ-ACK finally fed back may include a HARQ-ACK bit, which corresponds to a carrier without downlink HARQ transmission. When the maximum number of carriers supported by the UE is 5, existence of foregoing invalid HARQ-ACK bit will not bring significant impact on system performance. However, accompanying with increasing of maximum number of supportable carriers of the UE, e.g., the maximum number is increased to 32, negative impact resulted from the problem of invalid HARQ-ACK bit may be enlarged undoubtedly. Such problem is under discussion in the research of third generation partnership project (3GPP) Rel-13 eCA, which puts forward to feed back HARQ-ACK bit for an actually scheduled carrier. That is, feed back HARQ-ACK bit corresponding to a carrier with downlink HARQ transmission. To avoid inconsistent understandings of eNB and UE for number of HARQ-ACK bits to be fed back, e.g., eNB has scheduled downlink HARQ transmissions of N DL carriers, while UE only detects downlink HARQ transmissions of M DL carriers (M<N), 3GPP studies to count downlink assignment index (DAI) number of all the scheduled DL carriers. That is, sort DAI according to an ascending order of index number of each scheduled carrier. As shown in FIG. 21, the eNB has configured 16 carriers for the UE. The eNB has scheduled 5 carriers in subframe n, which are CC2, CC3, CC5, CC7 and CC15. DAIs in downlink control information (DCI) scheduling these 5 carriers are respectively 0-4.
It should be noted that, in the example illustrated with FIG. 21, the aggregated carriers are in the licensed frequency band. Carriers scheduled within one subframe are transmitted from subframe boundary simultaneously, that is, from #0th orthogonal frequency division multiplexing (OFDM) symbol of the subframe. Accompanying with increasing shortage of frequency resources, 3GPP starts to study how to transmit data on carriers in unlicensed frequency band. A LTE device may simultaneously work on carriers of licensed frequency band and unlicensed frequency band, by using CA or dual connectivity (DC). A significant difference between carriers in the unlicensed frequency band and carriers in the licensed frequency band is as follows. When a LTE device transmits a signal on a carrier of unlicensed frequency band, listen before talk (LBT) is needed. That is, the LTE device needs to monitor busy/idle state of the carrier in the unlicensed frequency band. Only when the carrier in the unlicensed frequency band is idle, the LTE device may transmit a signal on such carrier. Since the LTE device cannot accurately predict when the carrier in the unlicensed frequency band is idle, transmission of the LTE device on the carrier in the unlicensed frequency band is uncertain. That is, whether a signal can be transmitted in subframe n cannot be determined in advance. Meanwhile, to improve transmission efficiency of the LTE device on carriers in the unlicensed frequency band, transmission of the LTE device on carriers in the unlicensed frequency band is allowed to start from an intermediate position of a subframe. For example, PDSCH transmission may be started from subframe boundary, e.g., from #0th OFDM symbol, or from slot boundary, e.g., from #0th or #7th OFDM symbol, or from boundaries of more OFDM symbols, e.g., from #0th, #4th, #7th, #11th OFDM symbols. And then, when a LTE device transmits signals on multiple carriers within a same subframe, starting point of transmission time of each carrier may be different. For example, starting point of transmission time of carriers in the licensed frequency band is #0th OFDM symbol of subframe boundary. Starting point of transmission time of some other carriers in the unlicensed frequency band is a second slot, that is, #7th OFDM symbol of the subframe. As shown in FIG. 22, the eNB has configured 16 carriers for the UE. The eNB predicts to schedule 9 carriers in subframe n, which are respectively CC2, CC3, CC5, CC7, CC8, CC9, CC11, CC14 and CC15. Transmission of carriers in the licensed frequency band may be determined in advance, which may be started from #0th OFDM symbol. However, transmission of carriers in the unlicensed frequency band may depend on LBT. In FIG. 22, LBT has been completed by CC11 and CC15 before subframe n. Thus, transmission in subframe n may be determined before subframe n. And it may be determined to start transmission from #0th OFDM symbol. However, LBT has not been completed by CC7 and CC9 before subframe n. Subsequently, CC7 and CC9 continuously perform LBT in subframe n, until the LBT is completed before the second slot of subframe n. Thus, transmission in subframe n may be determined within subframe n. And it is determined to start transmission from #7th OFDM symbol. Since LBT has not been completed by CC8 and CC14 before the second slot, transmission of CC8 and CC14 cannot be initiated within subframe n. Subsequently, time of preparing for bits of DCI with DAI of each carrier may be earlier or later. Transmission time of DCI may be started from #0th OFDM symbol, or from #7thOFDM symbol. Thus, when numbering DAI according to an ascending order of carrier index, for a carrier with smaller carrier index number and later downlink transmission time, or for a carrier with greater carrier index number and earlier downlink transmission time, DAI number thereof cannot be determined. For example, regarding CC11 and CC15 in FIG. 22, carrier index number thereof is greater than that of CC7, CC8 and CC9. However, when generating DAI numbers of CC11 and CC15, whether it is necessary to reserve DAI numbers for carriers CC7, CC8 and CC9 cannot be determined, since eNB cannot determine whether carriers CC7, CC8 and CC9 can be transmitted within the same subframe. There is no ideal solution for such problem.
In addition, 3GPP also studies another type of DAI, which indicates the total number of all the downlink transmissions scheduled within current subframe. DCI of each downlink carrier scheduled within current subframe includes DAI. When all the carriers are in the licensed frequency band, whether all the carriers are transmitted within current subframe are determined in advance. Besides, all the carriers are transmitted simultaneously. When the aggregated carriers include carriers in the unlicensed frequency band, since whether carriers in the unlicensed frequency band can be transmitted within current subframe cannot be accurately predicted before current subframe, the total number of downlink carriers scheduled within the subframe cannot be accurately reflected by DAI in DCI, in which transmission of the DCI is started from #0th OFDM symbol of subframe boundary. As shown in FIG. 22, all the carriers transmitted within subframe n cannot be counted by DAI in DCI of CC2, CC3, CC5, CC11 and CC15, transmissions of which are started from #0th OFDM symbol of subframe boundary. That is, whether CC7, CC8, CC9 and CC14 can be transmitted within the second slot of subframe n cannot be counted. There is also no ideal solution for foregoing problem.
The foregoing problem also exists in a time division duplex (TDD) system.
In addition, accompanying with emergencies of new services, higher requirements have been put forward for time delay of wireless transmissions. Requirements of time delay cannot be satisfied by current subframe length of 1 ms, which is taken as the minimum time transmission interval (TTI). Thus, a shorter TTI, e.g., 1 subframe with 0.5 ms, or 1 OFDM symbol with 66.7 us, will be included in the study of 3GPP. And then, some carriers among multiple carriers may employ TTI of 1 ms, while some other carriers may employ a shorter TTI. Transmission of the shorter TTI may be started from an intermediate position of 1 ms subframe. Thus, current DAI indication cannot be applicable.
In the TDD system, the number of HARQ-ACK bits to be fed back by the UE in an uplink subframe n is determined by an HARQ-ACK time-frequency bundling window, a Downlink Assignment Index (DL DAI) carried in UL Grant (UG) of subframe n, number of carriers configured for the UE, and the transmission mode configured for each carrier, in which:
the HARQ-ACK time-frequency bundling window is determined by a TDD uplink-downlink configuration corresponding to a HARQ-ACK timing relationship followed by the HARQ-ACK fed back of the UE, denoting all downlink subframes on one carrier whose HARQ-ACK is to be fed back in subframe n. The indexes of the downlink subframes are denoted by n−ki, ki∈K, wherein the dimension M of the set K is referred to as the size of the time-frequency bundling window. At present, the set K determined by the LTE standard with respect to the HARQ timing relationships corresponding to different TDD uplink-downlink configurations is as shown in Table 2. For facilitating the description, the subframe set K corresponding to the time-frequency bundling window determined by the HARQ timing relationship of FDD is defined as {4}, M=1 at this time.
UL DAI denotes a maximum number of downlink subframes actually have downlink HARQ transmission in the time-frequency bundling window configured for each carrier of the UE. For each carrier configured for the UE, the number of downlink subframes need to feed back HARQ-ACK in subframe n is Bc=min(Mc, UL DAI), wherein min denotes an operation of obtaining a minimum value, Mc denotes the size of the time-frequency bundling window corresponding to the carrier c. If the transmission mode of the current carrier is one TB, the number of HARQ-ACK bits corresponding to this carrier is Oc=Bc, and each subframe corresponds to one HARQ-ACK bit. If the transmission mode of the current carrier is two TBs, the number of HARQ-ACK bits corresponding to the carrier is Oc=2×Bc, and each downlink subframe corresponds to two HARQ-ACK bits.
TABLE 2Table 2 set K: {k0, k1, . . . , kM−1} determinedby different HARQ timing relationshipsTDDuplink-down-linkconfig-Subframe indexuration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7,——4, 63——7, 6, 116, 55, 4—————44——12, 8, 7, 116, 5——————4, 75——13, 12, 9, 8,———————7, 5, 4, 11, 66——775——77—
In the TDD system, the HARQ-ACK bit sequence needs to be fed back by the UE is determined by a sum OUE of HARQ-ACK bits corresponding to all carriers. If OUE is not larger than 20, the HARQ-ACK bit of each carrier is arranged according to an ascending order of the carrier indexes to form the HARQ-ACK bit sequence to be fed back by the UE. Otherwise, if OUE is larger than 20, for all carriers whose transmission mode is two TBs, an “OR” calculation (i.e., spatial bundling) is performed to the two HARQ-ACK bits corresponding to two TBs of each subframe, to obtain one HARQ-ACK bit. For the carriers whose transmission mode is one TB, the HARQ-ACK bit corresponding to each subframe is remained unchanged. After the above processing, the HARQ-ACK bit of each carrier of the UE is arranged according to the ascending order of the carrier indexes to generate the HARQ-ACK bit sequence to be fed back by the UE.
It can be seen from the above description that, in the HARQ-ACK feedback mechanism defined by existing standard (LTE Release-12 and those before Release-12), the HARQ-ACK bit sequence finally fed back by the UE may include a HARQ-ACK bit corresponding to a downlink subframe which has no downlink HARQ transmission. For example, in the FDD system, whether or not there is downlink HARQ-ACK transmission on the carrier, the HARQ-ACK bit sequence fed back by the UE always includes an HARQ-ACK bit corresponding to that carrier. In the TDD system, the UE determines the number of downlink subframes having downlink HARQ transmission on each carrier according to Bc, but the value of Bc may be larger than the number of downlink subframes actually having downlink HARQ transmission in the time-frequency bundling window corresponding to the carrier. According to the current standard, the UE supports at most 5 carriers. Therefore, the existence of the nonsense HARQ-ACK bit does not have much impact to the system performance.
However, in order to further increase the downlink peak rate of the UE, it is well recognized by 3GPP member companies that the maximum number of carriers supported by the UE should be increased. The number of carriers supported by the UE will be increased to 32, wherein carriers on the unlicensed band may be included. With the increase of the number of downlink carriers supported by the UE, the absolute value of non-scheduled downlink subframes may increase accordingly. Therefore, the impact brought out by the nonsense HARQ-ACK bit is enlarged. In this case, if the current HARQ-ACK feedback mechanism is still utilized, the efficiency for feeding back information will decrease and finally affect the downlink peak rate of the UE, which contradicts to the initial objective of increasing the number of carriers. Therefore, in LTE Release-13, new bits are introduced, Total DAI and counter DAI (TS 36.212 Table 5.3.3.1.2-2). In the TDD system, the Total DAI indicates a total number of scheduled PDSCHs in all subframes and on all carriers from the first subframe to a current subframe in the HARQ-ACK time-frequency bundling window. The counter DAI indicates a total number of scheduled PDSCHs on all carriers before the current subframe in the HARQ-ACK bundling window and the scheduled PDSCHs on carriers from a carrier with minimum index to the carrier in the current subframe. In the FDD system, the total DAI indicates a total number of scheduled PDSCHs on all carriers in the current subframe, and the counter DAI indicates a total number of scheduled PDSCHs on carriers from a carrier with minimum index to the carrier in the current subframe. When the UE feeds back the HARQ-ACK, the number of HARQ-ACK bits is determined according to the total DAI, and the sequence of the HARQ-ACK bits is determined according to the counter DAI.
With the increasing shortage of spectrum resources, 3GPP begins the research on data transmission on unlicensed band. In LTE Release-13, an LTE device may operate on both the licensed band and the unlicensed band at the same time, via a carrier aggregation or double connection manner. An apparent difference between a licensed carrier and an unlicensed carrier is that, data transmission of the LTE device on the unlicensed band is based on listen before talk (LBT), i.e., the LTE device has to sense a busy/idle state of the unlicensed carrier. Only when the unlicensed carrier is idle, the LTE device is able to transmit on the carrier. Since the LTE device cannot accurately predict when the unlicensed carrier will be idle, the transmission of the LTE device on the unlicensed carrier is uncertain, i.e., it cannot be predicted that whether it can transmit in subframe n.